Due to well known reasons of effectiveness and limitation of disturbances due to power distribution mains, PWM switching converters may be preferable to traditional methods of regulation of AC motors and of other resistive-inductive loads, sampling the phase of the main sinusoidal voltage by controlling the conduction angle of a thyristor. These PWM switching regulators operate at relatively larger switching frequencies (in the order of tenths of thousands of Herz) than the relatively low frequency of the mains (typically of 50 or 60 Herz).
Nevertheless, even if PWM inverters designed for reducing the harmonic content of the currents absorbed by a nonlinear load have been developed, the need to respect the rules for limiting the harmonic content injected over distribution mains, mainly caused by the preliminary AC-DC conversion carried out by rectifiers that determine a relevant harmonic distortion of the current absorbed from the network, may impose the introduction of a power factor correction circuit (PFC) between the mains and the inverter.
The circuit disclosed in the European patent application EP 1,304,792 in the name of the same Applicant, discloses an AC-AC converter including a double chopper, capable of directly coupling the load to the sinusoidal voltage of the mains without carrying out a preliminary AC-DC conversion, thus avoiding issues of harmonic distortion of the current absorbed from the mains suffered by traditional AC-DC-AC converters,
For a correct functioning of the two choppers in case of inductive load, the circuit must be capable of powering the load starting from any level of the AC voltage of the mains and of allowing demagnetization of the inductance of the load through an efficient recirculation path of the discharge current.
As disclosed in the above identified patent application, the disclosure of which is hereby incorporated by reference in its entirety, if the chopper function (re: FIG. 1) is applied to the full sinusoidal waveform of the mains voltage, an AC current flows through the load. This current has the same shape of the waveform that would circulate if the load were connected directly to the electric mains. This means that if the shape of the original current is a sinusoid, the current, regulated through the PWM switching control of the power-on switches of the load and of the discharge switches of the inductance, is also sinusoidal.
Neglecting power losses in electronic devices, the circuit is such that the input (supplied) power S of the circuit equals the output (delivered) power:S=VAC(rms)IAC(rms)=VLOAD(rms)ILOAD(rms)                 and that the currents satisfy the following relation:        
            I              LOAD        ⁡                  (          rms          )                            I              AC        ⁡                  (          rms          )                      =            V              AC        ⁡                  (          rms          )                            V              LOAD        ⁡                  (          rms          )                                    wherein VAC and IAC represent the input voltage and the input current, respectively, and VLOAD and ILOAD are the output voltage and the output current of the converter, respectively, that are supplied to the load.        
The circuit functions as a converter, particularly as an AC-AC converter, and in practice as a classic transformer. Having fixed the power to be transferred, it is possible to have a load current larger than that IAC of the mains, because the output voltage is lower than the mains voltage.
Theoretically, there is no limitation on the type of load, it may work with any resistive, inductive and/or capacitive load, even with relevant out-of-phase angles. Nevertheless, the particular circuit topology and the way the two switches are controlled may call for a synchronization between the turning off of a switch and the turning on of its complementary switch.
Because the output filter and/or the load is inductive, the current that flows through the switches may not be interrupted instantaneously, otherwise large voltage swings capable of worsening the reliability and compromising the correct functioning of the system may be generated.
According to Lenz's law:
  ⅇ  =            -      L        ⁢                  ⅆ        I                    ⅆ        t            an abrupt variation of the current (that is if the time derivative of the current is relatively large) causes an induced electromotive force in the inductance L that may damage the switches. Therefore, the electric path of the load current may helpfully be switched from a switch to the other without interruptions.
On the other hand, a short superposition of turn on phases of the two switches, that causes a so-called “cross conduction,” would short-circuit the supply to ground. In a DC-AC converter that uses switches driven with complementary signals, dead time may be inserted between the turn off of a switch and the turning on of its complementary switch, during which both switches are turned off, to reduce cross conduction.
During the above dead times, the inductive currents flow freely through free-wheeling diodes normally integrated in the power devices. Because of the functioning of the AC-AC converter of the double chopper system, that may be defined as “bidirectional” because it works on both positive and negative half-waves, recirculation of discharge current of the reactive energy stored in the load and/or in the low-pass output filter during the disconnection phase of the load from the AC source may not occur through the intrinsic diode of the integrated structure of the power device, but may take place by turning on immediately the second switch, dedicated for this purpose.
This feature of a transformer circuit or of a switching AC-AC transformerless converter may cause designers to adopt compromises that restrict the field of possible applications in order to ensure a sufficient reliability.